Information-storage devices are essential to the functioning of modern society. Amongst the many types of information-storage devices, hard-disk drives (HDDs) occupy a prominent position, if not the dominant position, for the storage of information in the form of digital data, ranging from the storage of data in databases for governmental and financial institutions, to consumer applications such as desktop computers and set-top boxes for digital video. One performance metric of digital information storage in HDDs is areal density (AD), which is a measure of the amount of information that can be stored in a unit of area of the information-storage medium used to store information in the information-storage device, for example, an HDD.